The present invention generally relates to a disk array subsystem and its control method, and in particular relates to technology for optimizing the processing performance of a plurality of storage apparatuses storing data.
As one type of computer system, known is a disk array subsystem comprising a plurality of storage apparatuses. A disk array subsystem receives input/output commands from a host computer as an access source apparatus, or from other disk array subsystems. A disk array subsystem sends data retained in the storage apparatus to the access source apparatus, or sends data from the access source apparatus to one or more storage apparatuses provided to the disk array subsystem according to the foregoing input/output commands.
As this kind of disk array subsystem, there is the disk array subsystem disclosed in Japanese Patent Laid-Open Publication No. 2005-149173 (Patent Document 1). This disk array subsystem is attached to a plurality of storage apparatuses (disk drives, tape devices, etc.) using a fibre channel arbitrated loop (FC_AL: Fibre Channel Arbitrated Loop) as the connection interface.
Further, the specification of U.S. Pat. No. 7,035,952 (Patent Document 2) discloses a disk array subsystem newly adopting SAS (Serial Attached SCSI) as the connection interface of the storage apparatuses. Moreover, the specification of U.S. Published Application No. 2006/101171 (Patent Document 3) and Rob Elliott, ANSI INCITS T10 1760-D Serial Attached SCSI-2 (Draft) Revision8, 26 Jan. 2007, http://www.t10.org/ftp/t10/drafts/sas2/sas2r08.pdf (as of March 2007) (Non-Patent Document 1) disclose a SAS expander capable of forming a large-scale network by connecting expanders in a multistage tree structure. In addition, the topology of a SAS network configured from a plurality of SAS expanders is advantageous in that numerous storage apparatuses can be attached inexpensively with a single topology in comparison to a fibre channel arbitrated loop. The specification of U.S. Pat. No. 6,886,051 (Patent Document 4) and Non-Patent Document 1 disclose a discover process as a process for the SAS controller (initiator) to discover for the topology of the tree-structure switch network using the SAS expander.
Further, Rob Elliott, SAS-2 Multiplexing (Draft) Revison7, 6 Nov. 2006, http://www.t10.org/ftp/t10/document.05/05-381r7.pdf (as of March 2007) (Non-Patent Document 2) discloses multiplexing of transferring a plurality of data simultaneously with time-division multiplexing a plurality of logical links to a physical link of SAS. With this technology, when the maximum physical link rate of the SAS initiator is a higher link rate than the maximum physical link rate (transfer rate) of the storage apparatus, one high link rate physical link on the side of the SAS initiator is partitioned so that the input and output of data with two low link rate storage apparatuses can be controlled.
In a disk array subsystem adopting SAS as the interface for connecting a plurality of storage apparatuses (disk drives, etc.), there may be a configuration where a high link rate storage apparatus or expander adopting state-of-the-art SAS interface technology and a medium link rate or low link rate storage apparatus or expander adopting conventional SAS interface coexist. In this kind of connection configuration, storage apparatuses, expanders and SAS initiators having different maximum physical link rates will exist in the backend network of the disk array subsystem.
By employing the multiplexing disclosed in Non-Patent Document 2, when the maximum physical link rate of the SAS initiator is a higher link rate than the maximum physical link rate of the storage apparatus, one high link rate physical link on the side of the SAS initiator is partitioned so that the input and output of data with two low link rate storage apparatuses can be controlled. When the maximum physical link rates of storage apparatuses and the like are all of the same value, by setting the maximum logical link rate of all logical links to be the same as the maximum physical link rate of the storage apparatuses, the system performance of the overall backend can be utilized to the maximum extent.
Nevertheless, according to Non-Patent Document 2, in order to set the multiplexing, it is necessary to fix the number of logical links in the physical link during the link reset sequence as the initialization routine for establishing the physical link. Since the setting of the multiplexing is fixed at the time of initialization, the number of logical links configured in the physical link cannot be dynamically changed.
As described above, as a result of the restriction of not being able to dynamically set the multiplexing, when the maximum physical link rates of storage apparatuses and the like coexist, for instance, at 1.5 [Gbps], 3 [Gbps], and 6 [Gbps], in a case where the multiplexing is set to a slow storage apparatus of 1.5 [Gbps] to coincide with the maximum logical link rate, the logical link rate that can actually be input and output by a storage apparatus having a maximum physical link rate of 3 [Gbps] or 6 [Gbps] will be restricted to the maximum logical link rate of 1.5 [Gbps] set during the setting of the multiplexing. In the case of the same coexistence conditions as above, when the multiplexing is set to a high link rate storage apparatus of 6 [Gbps] to coincide with the maximum logical link rate of the logical link, a storage apparatus having a low maximum physical link rate of 1.5 [Gbps] or 3 [Gbps] will not be able to perform multiplexing and will not be able to use the bandwidth of the high link rate physical link of 6 [Gbps]. Thus, there is a restriction in that it is not possible to perform input/output from four storage apparatuses simultaneously with the four logical links set at a logical link rate of 1.5 [Gbps].
Accordingly, since the setting of the multiplexing is fixed, in a disk array subsystem attached to a plurality of storage apparatuses, it is not possible to set the backend network of the storage apparatuses to an optimal system performance when a plurality of maximum physical link rates of storage apparatuses coexist. Thus, there is a problem in that it is not possible to simultaneously utilize the maximum throughput performance of the backend of the disk array subsystem and the maximum physical link rate of the storage apparatuses.